The dialysis is a blood purification method which is used as a substitution method in case of renal failure. Apart from a kidney transplant, dialysis is the only and thus most important renal substitution therapy in case of chronic renal failure and one of the treatment options for acute renal failure. The term “dialysis” is to be understood as an exchange of substances through a membrane, with blood/plasma being present on the one membrane side and a dialysis solution being present on the other side of the membrane or flowing along it according to the countercurrent principle.
During a treatment, blood is pumped out of the patient via a patient-side access, is conveyed past the dialysis membrane in the dialyzer (filter) and returned to the patient in cleaned condition. In a hemodialysis or hemodiafiltration mode of the dialysis apparatus, any poisonous substances and uremic toxins (metabolic degradation products) and low-molecular substances (which are able to penetrate the membrane) are transported from the blood by concentration gradients (diffusion) and/or ultrafiltration (convection) via the membrane to the other filter side into the dialysis solution and are removed in this way. There is a constant flow of fresh dialysis solution through the dialyzer (approximately 500 ml/min).
Usually, the hemodialysis treatment is carried out for approximately 4 to 5 hours (dialysis overnight up to 8 hours) for every treatment and at least three times a week (depending on the body weight, renal residual function, cardiac output).
Patients who perform the hemodialysis at home avoid the problematic, longer treatment interval at the weekend and carry out the dialysis more frequently, as a rule every second day or even daily.
An important factor of influence on the quality of the dialysis is the blood flow. Generally speaking, the higher the blood flow, the better the result of the treatment. However, it may happen that problems occur in the dialysis shunt implanted in the patient used for taking blood and returning it, in particular if the blood flow is high in the extracorporeal system. The specific level of the blood flow at which these problems will occur depends on the quality of the patient-side access and on the blood flow through the shunt as well as its integrity (no stenosis or aneurysm).
During the dialysis, a so-called recirculation may occur in the shunt due to a change in the access, because of stenosis or if the flow in the shunt is too low. A recirculation causes the direct backflow of cleaned blood from the venous access directly into the arterial access of the dialyzer, whereby the effectiveness of the dialysis is significantly reduced. This is why it should/must be ensured that the dialysis performance does not decrease during the entire therapy and thereby an effective dialysis can be guaranteed.
In order to detect a recirculation in the shunt during the dialysis treatment, there already exist various control methods in prior art which are performed on the patient independently of the medical dialysis treatment method, the principles being illustrated in the following sections.